1. Field of the Invention
The present invention relates to an optical scanning device for use in an image forming apparatus.
2. Description of the Related Art
In image forming apparatuses, such as digital copiers, facsimiles, and laser printers, various optical scanning devices are used to scan a photosensitive member with a light beam. In a field of those image forming apparatuses, more and more apparatuses can form full-color images recently. For this reason, an optical scanning device capable of forming a plurality of scanning lines on a plurality of photoconductors at one time has been demanded.
Several schemes can be taken for a developing device satisfying such a demand. For example, a tandem-type developing device has been used in which four photoconductors each corresponding to yellow, magenta, cyan or black are arranged. As a low-cost optical scanning device suitable for such a tandem-type developing device, an oblique-incident-type optical scanning device has been used in which a light beam from a light source enters with an angle in a sub-scanning direction with respect to the normal line of a deflection plane of a deflector that deflects the light beam from a light source.
However, such an oblique-incident-type optical scanning device has a significant problem called “curved scanning line”. This “curved scanning line” is a phenomenon in which the path of a light beam is curved with respect to a main scanning direction on the external surface of a photoconductor as a scan target surface. The amount of occurrence of the scanning line is variably curved depending on the oblique incident angle of each light beam in the sub-scanning direction. If the scanning line is curved, a color-shifted toner image is formed, because a toner image is obtained by developing four latent images that are drawn with light beams to form four single-toner images and superposing the four single-color toner image.
Also, in the oblique-incident-type optical scanning device explained above, a light beam from the light source is caused to enter toward a rotational axis of a polygon mirror as a deflector. Therefore, when the light source is arranged on a line stretching from the optical axis of a scanning lens in the main scanning direction, the oblique incident angle (inclination of the light beam entering the deflector in the sub-scanning direction) increases so as to avoid interference with the scanning lens. For this reason, in the oblique-incident-type optical scanning device, the increase of the oblique incident angle increases the amount of occurrence of the curve in a scanning line explained above, and therefore the amount of occurrence of color shift also tends to increase.
Moreover, in the oblique-incident-type optical scanning device explained above, the light beam from the light source obliquely enters the deflector, and therefore the light beam enters with respect to the scanning lens in a twisted manner. This increases wave aberration and, in particular, the optical performance in image height peripheral to the luminous ray tends to significantly deteriorate. That is, in the oblique-incident-type optical scanning device, wave aberration tends to significantly deteriorate at a peripheral image height (near both ends of the scanning line) due to a light beam skew. When such wave aberration occurs, the spot diameter of the light beam is increased at the peripheral image height. Unless such a problematic increase in spot diameter of the light beam is resolved, “high-density optical scanning”, which has been strongly demanded in recent years, cannot be achieved. For this reason, in the oblique-incident-type optical scanning device explained above, an increase in image quality tends to be difficult due to wave aberration.
Furthermore, the increase in image quality demanded from the market requires a reduction in color shift. However, in the oblique-incident-type optical scanning device, a scanning line is curved. In particular, in an optical scanning device in which a plurality of light beams enter at different angles in the sub-scanning direction with respect to the normal line to the deflection plane, the magnitudes and directions of curves in scanning lines differ and, as a result, a large color shift occurs.
To suppress the curve in a scanning line described above, various schemes have been conventionally used.
In Japanese Patent Application Laid-Open Publication No. 10-73778 (hereinafter referred to as a first patent document), an optical scanning device is disclosed in which a plurality of rotational asymmetrical lenses are provided to an image-forming optical system in which an image is formed on the external surface of a photoconductor from a light beam deflected by a deflector and a generatrix shape connecting vertexes of child lines of lens surfaces of these aspheric lenses is curved in a sub-scanning direction.
In Japanese Patent Application Laid-Open Publication No. 2004-70109 (hereinafter referred to as a second patent document), an optical scanning device is disclosed in which an obliquely incident light beam is caused to pass outside of the axis of a scanning lens of an image-forming optical system and the aspheric amount of child lines of the scanning lens is formed on a plane varied along the main-scanning direction to align the positions of the scanning lines.
In Japanese Patent Application Laid-Open Publication No. 2006-11291 (hereinafter referred to as a third patent document), an optical scanning device is disclosed in which the number of reflection mirrors of an image-forming optical system is defined to match the directions of occurrence of curve in a scanning line, thereby reducing a color shift.
In Japanese Patent Application Laid-Open Publication No. 2006-72288 (hereinafter referred to as a fourth patent document), an optical scanning device is disclosed in which scanning lenses provided correspondingly to photoconductors of an image-forming optical system are each provided with at least one special tilt-decentered plane, which is a plane without a curvature in a sub-scanning direction and has a different tilt-decentered angle in the sub-scanning direction according to an image height.
In Japanese Patent Application Laid-Open Publication No. 11-14932 (hereinafter referred to as a fifth patent document), an optical scanning device is disclosed that adopts, as a scheme of correcting “a large curve in a scanning line” unique to an oblique incident type, a scheme of providing an image-forming optical system with “a lens having a lens surface on which an inclination unique to the lens surface in sub-scanning cross-section is changed in a main scanning direction so as to correct a curve in a scanning line”.
In Japanese Patent Application Laid-Open Publication No. 11-38348 (hereinafter referred to as a sixth patent document), an optical scanning device is disclosed that adopts a scheme of providing an image-forming optical system with “a corrected reflection plane having an reflection plane obtained by changing an inclination unique to the reflection plane in sub-scanning cross section into a main scanning direction so as to correct a curve in a scanning line”.
However, the lens having a “lens surface in which a generatrix shape connecting vertexes of child lines is curved in a sub-scanning direction” disclosed in the first patent document solves various problems by curving the generatrix, and an individual scanning lens is required to support each incident light beam. Therefore, if such a lens is simply applied to a tandem optical system, the number of the types of scanning lenses is increased, thereby increasing the number of types of component configuring an optical scanning device, that is, an image forming apparatus.
Also, the lens disclosed in the first patent document has a curvature in a sub-scanning direction. Therefore, if a position from which the light beam enters the lens is shifted in the sub-scanning direction due to influences of assembling error, processing error, environmental fluctuations, and other factors, the shape of the curve in a scanning line is changed upon reception of the influence of refracting power of the lens in the sub-scanning direction. This poses a problem of occurrence of color shift without achieving an initial (or designed) effect of suppressing a color shift in a color image.
In the optical scanning device disclosed in the second patent document, the correction is exemplarily made with one scanning lens. Although the correction of a curve in a scanning line can be made, deterioration in light beam spot diameter due to an increase in wave aberration, which will be explained further below, is not mentioned.
In the optical scanning device disclosed in the third patent document, a color shift is reduced by providing an odd-number difference between the number of reflection mirrors for light beams entering from an upper side of a polygon mirror as a deflector and the number of reflection mirrors for light beams entering from a lower side thereof, thereby aligning the directions of the curve in the scanning lines. However, since an anamorphic aspheric lens is used as a scanning lens, the amount of occurrence of curve in a scanning line is not mentioned.
Also, polygon mirrors adjacent to each other have the same number of reflection mirrors for two light beams entering from the upper side of these polygon mirrors, and also have the same number of reflection mirrors for light beams entering from the lower side of these polygon mirrors. Therefore, the layout disadvantageously tends to be restricted.
In the optical scanning device disclosed in the fourth patent document, the curve in a scanning line is excellently corrected by using a special tilt-decentered plane. However, the second lens for each light beam has an asymmetrical shape in a main scanning direction. Therefore, when the optical scanning device is used for a one-side scanning system, for example, the number of types of lens is increased to four, thereby increasing a development period. Thus, the number of types of component configuring the optical scanning device, that is, an image forming apparatus, is increased.
Furthermore, in the optical scanning devices disclosed in the first to sixth patent documents, a large curve in a scanning line, which is a characteristic to the optical scanning device of an oblique incident type, can be excellently corrected, but wave aberration explained above cannot be sufficiently corrected.
In particular, in the optical scanning device disclosed in the first patent document, when a plurality of light beams toward different scan target surfaces are caused to enter the same lens, various problems can be solved for one light beam by curving a generatrix shape. However, for other light beams, it is difficult to reduce a curve in a scanning line and wave aberration. Moreover, also in correction of wave aberration, as for a plane with a curvature, the skew state of a light beam is significantly changed due to fluctuations of the incident light beams, and therefore it is difficult to stably achieve a light beam having an excellent spot diameter.